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4/7/15 11:54 AM USAF Test Pilots School Vets Safer Adaptive Flight Controller | Technology content from Aviation Week Page 1 of 4 http://aviationweek.com/technology/usaf-test-pilots-school-vets-safer-adaptive-flight-controller Store Subscribe RSS eBulletin Advertise Contact Us Apr 6, 2015 Tweet Tweet COMMENTS 0 Advertisement USAF Test Pilots School Vets Safer Adaptive Flight Controller Guy Norris Aviation Week & Space Technology An adaptive flight controller that could help pilots save a critically damaged or out-of- control aircraft is being proposed for possible commercial development following a rigorous evaluation by U.S. Air Force Test Pilots School (TPS) students here, using Calspan’s variable-stability Learjet 24 test aircraft. The L1 controller is designed to automatically intervene in the case of control problems, immediately reconfiguring the flight control system to compensate for degraded flying qualities from mechanical failure or battle damage to a control surface, or even the unintended result of shifting center-of-gravity inflight for better cruise performance. Acting as a backup to the standard flight control system, the L1 is designed to provide safe, predictable, reliable and repeatable responses that would free up pilots to deal with the emergency and further compensate for reduced performance. In development for more than a decade by researchers at the University of Illinois at Urbana- Champaign, the L1 controller architecture differs from most previous approaches to adaptive control systems. Until now, the standard has been gain-scheduled control systems in which the flight control computer selects the appropriate preprogrammed gains to suit current flight conditions and vehicle configuration. However, the L1 works in real-time to predict transient behavior and estimates lumped uncertainties, rather than every individual parameter that can affect system dynamics; it compensates for them within the bandwidth of a control channel. The L1 controller comprises three blocks: a state predictor, a fast estimation scheme and a control law. The fast estimation scheme includes a state predictor and a fast estimation law which together approximate the dynamics of the aircraft to generate estimates of the uncertainties. These estimates are provided as input to a bandwidth-limited filter which generates a control signal to the flight control system. Advertisement 2015 Media Materials Your source for 2015 media kits, editorial planners rates and specs for AW&ST, ATW B&CA and more! ADVERTISE View Edit Nodequeue Clone HOME > TECHNOLOGY > USAF TEST PILOTS SCHOOL VETS SAFER ADAPTIVE FLIGHT CONTROLLER SHARE SHARE 51 Recommend Recommend Step Inside... NOW AVAILABLE ONLINE Technology Every Monday: the l from Aviation Week Space Technology.. Special Topics SEE ALL SPECIAL TOPICS Optimizing Engin Through the Lifec Sponsored by: GE Avaition... [email protected] [email protected] Pending Pending Create content Create content Administer Administer Access Aggregated Content Article Audio Author Binary poll Block Blog Entry Blog home page Blogger Profile Brand Category Comment Snippet Curated Search Curated Topic Data Table Datasheet Downloadable Event Event Product Expert FAQ Gallery Landing Page Link List Magazine Issue Newsletter Newsletter Import Page Physical Product Podcast Press Release Product Product kit Question and Answer Ranking poll Sponsored content Subscription Third Party Product Technology Commercial Aviation Space Defense Business Aviation MRO Events Aviation Week Intelligence Network Aviation Week W parmalee@aviationwe My Acco
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4/7/15 11:54 AMUSAF Test Pilots School Vets Safer Adaptive Flight Controller | Technology content from Aviation Week
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USAF Test Pilots School Vets Safer Adaptive FlightControllerGuy Norris Aviation Week & Space Technology
An adaptive flight controller that could helppilots save a critically damaged or out-of-control aircraft is being proposed for possiblecommercial development following arigorous evaluation by U.S. Air Force TestPilots School (TPS) students here, usingCalspan’s variable-stability Learjet 24 testaircraft.
The L1 controller is designed toautomatically intervene in the case of controlproblems, immediately reconfiguring theflight control system to compensate for
degraded flying qualities from mechanical failure or battle damage to a control surface, or even theunintended result of shifting center-of-gravity inflight for better cruise performance. Acting as abackup to the standard flight control system, the L1 is designed to provide safe, predictable,reliable and repeatable responses that would free up pilots to deal with the emergency and furthercompensate for reduced performance.
In development for more than a decade by researchers at the University of Illinois at Urbana-Champaign, the L1 controller architecture differs from most previous approaches to adaptivecontrol systems. Until now, the standard has been gain-scheduled control systems in which theflight control computer selects the appropriate preprogrammed gains to suit current flightconditions and vehicle configuration. However, the L1 works in real-time to predict transientbehavior and estimates lumped uncertainties, rather than every individual parameter that can affectsystem dynamics; it compensates for them within the bandwidth of a control channel.
The L1 controller comprises three blocks: a state predictor, a fast estimation scheme and a controllaw. The fast estimation scheme includes a state predictor and a fast estimation law which together
approximate the dynamics of the aircraft to generate estimates of the uncertainties. These estimates are provided
as input to a bandwidth-limited filter which generates a control signal to the flight control system.
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4/7/15 11:54 AMUSAF Test Pilots School Vets Safer Adaptive Flight Controller | Technology content from Aviation Week
Page 2 of 4http://aviationweek.com/technology/usaf-test-pilots-school-vets-safer-adaptive-flight-controller
Tests of the controller in Calspan’s variable-stability Learjet included dealing with roll-coupled yaw while simulating a lifting body and high adverse aileron yaw whilereplicating an F-100 Super Saber. Credit: Calspan
Unlike other adaptive controllers that use the estimated values of uncertain parameters directly ascontrol gains, the L1 system decouples the estimation loop from the control loop. Thisarchitectural change, which is achieved through the presence of the bandwidth limited filter in thecontrol structure, avoids the high-gain response of earlier adaptive systems, making it safer, robustand easier to certificate, say developers.
“We can quantify ahead of time the transient steady-state performance specification androbustness margin in an aircraft and set guidelines for the trade-off between performance androbustness and the adaption piece,” says TPS Flying Qualities Master Instructor Chris Cotting. “IfI have a stability augmentation system I can make it incredibly stable but not very maneuverable,or very maneuverable but not very stable. L1 [allows you] to make those kinds of trades with thisadaptive controller.”
L1 development from theory to flight test has been led by Naira Hovakimyan, a professor ofmechanical science and engineering at Illinois, who worked on the concept under Air Forcefunding between 2004 and 2008 at Virginia Tech with postdoctoral fellow Chengyu Cao (now atthe University of Connecticut). The TPS evaluation was the first time the controller had beenflown on a manned aircraft and was a vital step toward implementing L1 as a flight safety system,says Hovakimyan. “This is just a pure flight control system like an autopilot, but you can augmentit with additional features such as envelope protection, different pilot interfaces and so on.” NASAis funding that endeavor, “and now we are focused on revolutionizing the fight control system ofcommercial aviation.”
Although studies into adaptive flight control systems in the U.S. go back to the X-15 program inthe late 1950s, the L1 originated with lessons learned from the Boeing/Air Force ResearchLaboratory X-36 Restore program, says Hovakimyan. Under this effort, which ran from 1996 to1999, researchers attempted to stabilize an unstable, tailless unmanned air vehicle (UAV) withseveral simulated control-system failures.
“Predictability is the key word,” Hovakimyan says. “In the Restore program, when they tested itwith adaptive controllers they didn’t know how to tune it. It was stable but not predictable andevery time they got different transients. Adaptation can help, but the architecture itself haddeficiencies. It was good for slow adaptation but it was not correctly structured to maintainrobustness in the presence of fast adaptation.” Subsequent studies focused on development of afast-adapting controller that could also maintain robustness. “That’s what we nailed downarchitecturally,” she adds.
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4/7/15 11:54 AMUSAF Test Pilots School Vets Safer Adaptive Flight Controller | Technology content from Aviation Week
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The L1 was evaluated by the pilot in the right seat using a joystick. The left seat, with theconventional Learjet controls, is for the safety pilot. Credit: Guy Norris/AW&ST
The L1 was flown for the first time in 2006 in the Rascal UAV at the Naval Postgraduate Schooland again starting in 2009 when NASA evaluated the controller on the AirSTAR DynamicallyScaled Generic Transport Model research aircraft. In 2011, L1 was tested on the Simona motion-based research simulator at the Delft University of Technology in the Netherlands. Versions of theL1 controller are used in marine autopilots and industrial machines, and are being studied forUAVs and missiles.
Flight tests were conducted at Edwards AFB under one of the school’s Test Management Projects,which provide students and staff with the opportunity for short-duration, real-world flight tests.Industry, academia or other military customers provide the research concept and any requiredfunding while the school provides test aircraft, a team of test pilots and engineers as well asresources ranging from the Edwards airspace to flight-test data and analysis (AW&ST Dec. 1/8,2014, p. 57).
Testing was conducted over two weeks in February and March on a Calspan Learjet 25D modifiedwith a variable stability simulator (VSS). This feature changes the apparent stability of the aircraft,allowing it to be used as an inflight simulator of other aircraft types. The L1 was overlaid on top ofthe VSS, which simulated various aircraft in up-and-away flight at 250 kt. and 15,000 ft., andpowered approach with gear down and flaps at 20 deg. If the Learjet approached dangerous flightconditions, a series of safety trips disengaged the VSS, returning control to the safety pilot in theleft seat.
“The VSS puts in the good or bad dynamics we want, and the L1 was designed to restore thereference set,” says Cotting. “If we make the VSS better than the L1, it will degrade it to thereference model, or if we make it worse, the L1 will make it better,” he adds. Students assessedflying and handling qualities over 14 hr. of test time at the two test conditions, as well as measuredthe robustness of the adaptive controller. Data also was collected for offset approaches andstraight-in landings.
Specific failures—applied either individually or in combination—included reduced pitch damping,aft center of gravity for neutral static longitudinal stability, reduced yaw damping, high adverseand proverse aileron yaw, reduced roll damping and a coupled roll-spiral mode. Duringsimulations of a lifting body, the L1 overcame lateral direction and oscillation control issues, andwhile representing an F-100 Saber at high angles of attack, the system enabled normal rollcontrol.
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4/7/15 11:54 AMUSAF Test Pilots School Vets Safer Adaptive Flight Controller | Technology content from Aviation Week
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